The brief
This S5 came in with the check engine light on and a flat spot in the power. We ran the Audi scan tool on it, because the live data behind the code is what tells you whether a sensor is faulty or just reporting a problem somewhere upstream. The intake manifold on these engines has a set of swirl flaps inside it, driven by an actuator, that change how air enters the cylinders to suit the revs. The flaps and the linkage carbon up and wear over the years, and the actuator that moves them can pack up, so the flaps stick or stop reporting their position. The engine management sees that, throws a code, and the engine runs flat because the intake isn't doing what it should. On a lot of these the flap mechanism isn't a separate part, so the fix is a new intake manifold.
The diagnosis
The Audi scan tool pulled the stored fault and the live data showed the intake manifold flap mechanism not reporting the right position, the actuator and the flaps worn, which is the light and the flat spot. The injectors, the sensors and the rest of the engine checked out. That's an intake manifold replacement, the whole unit with the flap mechanism in it, plus fresh gaskets, rather than trying to free up carboned, worn flaps that would seize again.
The work
The old intake manifold was removed, the ports and the intake side cleaned up, and a new genuine Audi-spec intake manifold fitted with fresh gaskets, every fastener torqued to the manual figures and the actuator connected and checked. The fault was cleared, the engine's adaptations reset so it relearns on a manifold that works, and the air filter checked. A road test confirmed the light stayed off, the idle steady, and the power back with no flat spot.
The outcome
No check engine light, a steady idle, full power back with no flat spot, smooth running through the rev range, and the fuel trims in range. The S5 went home running the way it should. Worn flaps only seize harder, so replacing the manifold as a unit and resetting the adaptations fixed it properly, with no carboned linkage waiting to jam again.
